The present invention pertains to a power amplifier and envelope correction circuitry associated therewith. Power amplifiers, and especially semiconductor power amplifiers, can be very non-linear so that providing a preferred pulse envelope at the output thereof is extremely difficult. This is especially true if the duty cycle, or sequence, of a pulse train is not constant. Further, in many applications the peak pulse power may be relatively large so that temperature changes and the like within the circuitry of the power amplifier have a substantial and adverse effect.
As a specific example, TACAN ground station transponders, which provide distance, azimuth, and station identification information to properly equipped aircraft, must be capable of producing pulse trains wherein the time between pulses varies radically but the amplitude and shape of the pulses is closely controlled. In general, the shape of the pulse should be a smooth bell curve, or Gaussian wave form, to minimize the frequency spectrum occupied by the radiated energy so as not to interfere with adjacent channels in the TACAN system. Any variations in the shape of the output pulse may cause spectrum spreading, and may result in interference with adjacent TACAN channels. Any variations in the amplitude of the output pulse may cause erroneous navigational information.
In the prior art, the output pulse shape and amplitude is controlled with an analog loop around the power amplifier. The components of these circuits have many parts with carefully controlled values, and are expensive and difficult to manufacture. Further, to reduce the requirements of the analog loop the power amplifier must be constructed as linear as possible. Since the power amplifier, in a TACAN system for example, is an RF amplifier, the components are RF devices which are generally expensive and difficult to manufacture.
A typical example of prior art structure is disclosed in U.S. Pat. No. 4,237,555, entitled "Automatic Modulation System", issued Dec. 2, 1980. This structure includes several analog feedback loops and apparatus for generating a pedestal in the feedback.
While the TACAN navigational system is utilized herein as a specific example, it will be understood by those skilled in the art that a great variety of applications exist wherein precise control of the output of a nonlinear amplifier is required. Further, while a power amplifier is specified it will be clear to those skilled in the art that the present envelope correction circuitry can be manufactured independent of the amplifier.